1. Field of the Invention
The present invention relates to an optical transmission system, and more particularly to an optical transmission system which transports optical signals using wavelength-division multiplexing techniques.
2. Description of the Related Art
Wavelength-division multiplexing (WDM) techniques are widely used in today's optical communications infrastructures. A WDM system carries many transmission signals (e.g., forty to one hundred waves) over a single fiber-optic medium, with different optical wavelengths assigned to different channels. Long-haul WDM systems employ a number of optical amplifiers to boost optical signals. They include: preamplifiers, post-amplifiers, and inline amplifiers. Preamplifiers are placed at the front end of each node to receive and amplify WDM signals received from the preceding node. Post-amplifiers are placed at the output of each node to increase the power of processed WDM signals before they are transmitted to the next node. Inline amplifiers are intermediate boosters placed at one or more points on a repeater section between a sending node and a receiving node.
The input dynamic range of optical amplifiers designed for a WDM system is typically 7 to 12 dB. Large input signals above this range would worsen the noise factor (NF) or produce an increased gain tilt over multiple signal wavelengths. To avoid those problems, optical signals have to be adjusted to an appropriate power level to meet the dynamic range requirement of an amplifier before they are supplied thereto.
The fiber-optic transmission lines of a WDM network may be segmented into a plurality of sections separated by intermediate repeater devices. Each repeater section has a different amount of signal power loss, depending on the length and type of the fiber that is used. It is therefore important to set the input power level of preamplifiers and inline amplifiers, which receive and amplify optical signals to help them propagating over a transmission line. Such input level adjustment has conventionally been achieved by inserting an optical attenuator with a fixed attenuation ratio, or by manually tuning the attenuation ratio of a variable attenuator that is employed in each network node.
FIG. 20 shows signal power level adjustment in a conventional WDM system, focusing on its essential elements for explanatory purposes. The illustrated system includes a fiber-optic transmission line that connects two end nodes 110 and 120 via a repeater node 130 disposed between them. The nodes 110 and 120 are optical add/drop multiplexers (OADM), which extract optical signals with particular wavelengths out of the incoming WDM signals and insert new optical signals to outgoing WDM signals, which are referred as “drop” and “add” functions, respectively. Attached to the leftmost node 110 in FIG. 20 is a transponder 111, which converts wavelengths or interface of signals supplied from a tributary to add them into the outgoing WDM optical stream. The repeater node 130 contains an attenuator (ATT) 131 and an inline amplifier (ILA) 132. The receiving node 120 has an attenuator 121, a preamplifier 122, and a post-amplifier 123.
As part of a system setup process, optical attenuators have to be tuned to provide an appropriate level of signal reduction. The attenuator adjustment needs some amount of input light, and it is typical to use add-channel optical signals from a transponder for this purpose. In the example of FIG. 20, the transponder 111 serves as a light source for attenuator adjustment, delivering add-channel light through the node 110. To start up this conventional WDM system, a service engineer conducts a power level measurement at point p1 in the repeater node 130. This measurement yields the intensity of the add-channel optical signal received over the first repeater section A1. Based on the measurement result, he/she then adjusts the attenuator 131 so that the observed signal level at point p1 will match with a specified input level that the inline amplifier 132 requires.
In the case the attenuator 131 is a fixed attenuator, the service engineer has to remove it (and give a bypass to make optical signals go through) before conducting the above measurement at point p1. The measurement permits him/her to determine how much attenuation is required and thus to choose and insert an appropriate fixed attenuator that reduces the signal strength by the required amount.
The attenuator 131 will be allowed to sit there if it is a variable attenuator. In this case, the service engineer first sets up the attenuator 131 with a minimum attenuation (so that the signal can just go through it) before measuring the signal power level at point p1. He/she then adjusts the attenuator 131 manually to bring the signal to a desired power level.
The same adjustment procedure is applied to another point p2, at which an optical signal coming over the next repeater section A2 is observed. The service engineer measures the power level of this signal and configures the attenuator 121 in the receiving node 120, so that the observed signal level at point p2 will match with a required input level of the preamplifier 122.
With both attenuators 131 and 121 adjusted, the inline amplifier 132, preamplifier 122, and post-amplifier 123 are now receiving add-channel optical signals with intended amplitudes. The amplifiers are thus allowed to enter a specified control mode, which may be automatic level control (ALC) mode or automatic gain control (AGC) mode. ALC mode regulates the output power level of an optical amplifier, canceling the effect of input power fluctuations. In ALC mode, the power of each wavelength is maintained at a constant level, meaning that the target power level is a function of the number of wavelengths. AGC mode, on the other hand, tries to maintain the gain of an optical amplifier at a given value. Every optical attenuator in the system is adjusted in this way, and every optical amplifier is brought to a prescribed operational state, whereby the entire system can start to run.
A more specific example of a conventional technique is disclosed in the Japanese Patent Application Publication No. 2003-174421, in which paragraphs [0020] to [0119] and FIG. 6 are particularly relevant. This literature proposes a technique to stabilize the optical amplifier output by adjusting an optical attenuator placed at the input end of the amplifier, such that the output power will be maintained at a particular level specified through a supervisory control signal.
One thing to consider about attenuator adjustment is that it is a series of manual tasks performed by service engineers. This fact means the following problems: First, such tasks are not error free. Second, they need knowledge and skills. Third, they are quite time-consuming since adjustment should take place at each individual node.
To address the problems enumerated above, some systems use remotely-controllable variable attenuators. Those attenuators can communicate with a remote site through the use of appropriate protocols, such as the Transaction Language 1 (TL1), a set of network management commands developed by Telcordia Technologies Inc (formerly Bellcore). This type of variable attenuators permit an engineer to monitor their input power levels and set attenuation depths by sending commands and receiving response messages.
Although the workload of attenuator adjustment can be reduced by the use of such remote control facilities, service engineers are still burdened with some amount of manual tasks that they have to perform through a terminal console. Also, the remote attenuator control requires the system to provide an additional interface environment for that purpose.
Another issue to consider here is how to supply optical power for attenuator adjustment. As mentioned earlier, one option for this is to use an add-channel signal from a transponder. In this case, however, an additional system has to be organized for startup processing using transponders as a light source. Since telecommunications carriers direct their efforts toward quick service provisioning at minimum cost, their recent approach is to install transponders on an “on-demand” basis. That is, they construct a new WDM system without transponders connected. When a specific service demand arises, they attach transponders to the existing multiplexer/demultiplexer equipment to provide service. This means that there are no transponders when we need them to use as an optical power source for attenuator adjustment.
Another difficulty lies in the fact that some systems have protection links to back up working links for enhanced availability and reliability. Protection links carry no optical signals in normal circumstances, as mentioned in paragraph [0102] of the Japanese Patent Application Publication No. 2003-174421. This means that we have to prepare some other light source for adjustment of attenuators on a protection link.
For the above reasons, optical transmission systems need improved startup functions that do not require signal light from transponders. It is, however, costly and inefficient to employ a separate light source with a variable wavelength only for that purpose.